Devices for rolling-up and unrolling a fabric cover sheet to respectively uncover or to cover a top opening of a truck box are well known in the prior art as disclosed, for example, by Dahlman U.S. Pat. No. 2,976,082 and by Sargent U.S. Pat. Nos. 3,384,413 and 3,785,694, showing crank-actuated rolls for reeling up or unreeling the cover by traversing the roll from one long edge of the box to the other over an array of roof-shaping arches spanning the width of the box. Such devices however lack an effective and simple means of maintaining the cover taut and in place at highway speeds; one device employs springs which are stretched to bias the roller and tension the sheet as the box is covered, and employs wire holders to secure the roller in place. Other devices make use of a series of straps spaced along the sheet, which require to be made taut in turn and secured to the box sidewall as by tying or clamping. A particularly unsatisfactory tendency of such prior covers to admit dust or rain under the sheet at highway speeds has necessitated overcoming the problem of assuring a good seal between the box wall edges and the cover sheet under all weather and road conditions.
There has been disclosed in Dimmer et al U.S. Pat. No. 4,302,043 a crank-operated bar which when turned beyond the fully-unwound roll position depends from an overhanging sheet portion that overlies a metal plate fixed on the top flat surface of a frame member in the plane of the box opening, the plate being angled downwardly and outwardly. Further unwinding of the bar causes the sheet-covered bar to cramp against the undersurface of the plate when sufficient torque is exerted by the crank arm. While such apparatus provides a drip edge, the sheet often requires to be re-tensioned en route due to the strong frictional clamping caused by the contiguous relation of the sheet and the plate, and excessive wear is experienced along the edge of the plate over which the sheet is sharply folded.